The worldwide growth in the use of mobile phones, digital scanning apparatus, medical instruments, and security and surveillance devices etc. containing cameras is driving the demand for improvements and developments of existing camera technology.
To meet demands from a user perspective (such as larger focal range, greater operating optical versatility and focusing speed), and from a manufacturing perspective (such as reducing the material processing requirements/limitations for device design and cost of manufacture), further improvements to existing optical device designs are required. Improvements to the design of existing micro-tunable lenses for example are critical to ensure that the devices are more readily suitable for its target application in high-volume consumer electronic devices (e.g. cameras within mobile-phones, PC's, digital scanners, etc.).
An example of an improved micro-tunable lens is disclosed in the patent EP 2115500 with the title “Flexible lens assembly with variable focal length” having a flexible lens body 10 in a cavity bounded by a sidewall 11 and a first transparent cover 13, and a second transparent cover 14, wherein both covers 13, 14 are in contact with respective surfaces of the lens body 10. Piezoelectric elements 12 are shaping the lens body 10 when activated, thereby adjusting for example the focal length of the lens assembly.
There are some examples of manufactured exemplars of the above referenced flexible lens assembly that are 0.4 mm thick. It is also possible to achieve even smaller exemplars of this design. The movement of the lens body shape, when the piezoelectric elements are shaping the body, is in the μm range when adjusting the focus length from infinity to for example 10 cm. Therefore, this design is an example of an extremely slim design configurable to be used in a camera module for example. Embodiments of the flexible lens assembly according to the patent EP 2115500 are sold under the trade name TLens™. An example of embodiment of a TLens™ chip is disclosed on the web page http://www.polight.com/tlens-13.html.
The referenced example of an improved micro-tunable lens utilizes piezoelectric electrodes of a conventional design, i.e. an upper and a lower metal layer with a ferroelectric layer in between the two metal layers (ref. FIG. 2). This type of piezoelectric electrode has proved to be able to provide autofocus capability for camera systems, and is regarded as a replacement technology for the more common Voice Coil Motor (VCM) systems in use in mobile phone cameras. It is further known that the same type of piezoelectric electrode configurations can be used in Optical Image Stabilization systems, for example as disclosed in the European Patent application EP 08712670.
From a manufacturing point of view, a piezoelectric electrode with fewer layers would be beneficial. In prior art it is known an electrode configuration named Interdigitated Electrodes that can be manufactured with for example respective positive and negative electrodes that are located spaced apart from each other on top of a ferroelectric layer surface on top of an insulating substrate (ref. FIG. 1).
The patent application US 2012/0053393 by Dominik Kaltenbacher et. al. discloses a sound transducer for producing sound waves, which can be inserted into an ear. FIG. 6 of this publication illustrates a piezoelectric electrode configuration with parallel positive and respective negative electrodes spaced apart on top of a piezoelectric layer which rest on top of a sound membrane member. FIG. 6 also discloses how the electric field lines between the electrodes penetrates the ferroelectric (piezoelectric) layer, and the piezoelectric effect manifests itself when, for example, alternating voltages are applied on the electrodes, and the length of the piezo crystals are respectively lengthened or shortened, thereby producing a bending upwards or downwards of the surface, which then are mechanically transferred to the sound membrane member which then can reproduce sound. However, since the electrodes are positioned on top of the ferroelectric layer, there will be no electric field directly under the surface of the electrodes facing the ferroelectric layer. Therefore, the electric field lines penetrating the ferroelectric (piezoelectric) layer are shaped with a curved shape in between the electrodes. Therefore, there will not be any suitable piezoelectric effect just underneath the electrodes themselves. The areas in which this phenomenon occurs are often referred to as “dead zones”. The effect is that the bending of the surface (i.e. the sound membrane in this example) is non-uniform. When reproducing sound, this is not a problem. It is the vibration of the membrane which is important, as known to a person skilled in the art.
U.S. Pat. No. 5,451,769 disclose a high speed photo detector that in an example of embodiment as depicted in FIG. 3 is arranged with an Interdigitated Electrode configuration comprising two electrodes of opposite polarity, wherein each respective electrode comprises a radially oriented electrode part wherein circle shaped parts are connected and which constitutes rings around the centre of the photo detector. However, this configuration has also dead zones, but the specific application of the Interdigitated Electrode does not affect the performance of the application as part of a photo detector.
Therefore, if an Interdigitated Electrode configuration for example should replace a conventional piezoelectric actuator in the example of a micro-tunable lens as disclosed in the patent EP 2115500 referenced above, the optical quality would probably be degraded significantly due to the possible dead zone effect of the Interdigitated Electrode configuration (uneven shaping of the lens body).
However, the possible benefits and advantages that could be achieved by utilizing an Interdigitated Electrode configuration on top of a flexible lens body would not only reduce the thickness of the piezoelectric actuators because of fewer layers, but also, for example with reference to the examples of embodiments disclosed in EP 2115500, because the bendable glass cover 13 can serve as an insulating layer, wherein a ferroelectric material can be deposited on the surface followed by deposition of the electrode configuration on top of the ferroelectric layer. This simplifies the manufacturing of the micro-tunable lens significantly. Further, it is known that the bending force of an Interdigitated Electrode configuration may provide an increase in the possible maximum bending forces.
According to an aspect of the present invention, a micro-tunable lens with Interdigitated Electrode configured piezoelectric actuators may achieve improved optical performance and versatility if the electrodes are arranged in a configured array providing a mitigation of the dead zone phenomena of Interdigitated Electrodes.
Hence, an improved piezoelectric actuator configuration would be advantageous, and in particular it would be more efficient to manufacture a micro-tunable lens which has an improved piezoelectric actuator configuration, and which would provide the following examples of improvements:                i) Designing piezoelectric actuator element(s) with an Interdigitated Electrode pattern configurable to provide a bending force distribution across a flexible lens surface area of a pre-determined and desired uniformity. Such a bending force may be configurable to be homogenous in order to provide an increased focal length compared to prior art solutions. It is also within the scope of the present invention that the Interdigitated Electrode pattern is configurable to provide non-homogeneous (distorted) bending force distribution in order to compensate optical effects and aberrations (or lens faults), or otherwise due to specific device application requirements.        ii) Designing piezoelectric element(s) and electrode configurations with minimal piezoelectric layer thickness and/or operating voltage requirements. These are critical design issues.        iii) Designing piezoelectric element(s) and electrode configurations within a micro-tunable lens with as few layers as possible. This reduces the number of manufacturing steps, materials and components and therefore the associated cost.        